This invention relates to optical thin films, and particularly to the preparation of relatively stress-free optical thin films by deposition of successive layers under substantially isothermal conditions.
The term "optical thin film", as used herein, refers to a film with unique optical properties, capable of functioning, for example, to reflect radiation within a specifically defined wavelength region; for example, the 700 nm. region. Such films are in common use in many environments. For example, they may be used as filters in lighting applications, as windows for radiation in a specific wavelength region, as non-reflective coatings on eyeglasses and as heat-reflecting coatings on gas turbine blades to reduce heat absorption thereby.
It is known to produce optical thin films comprising successive layers of silicon dioxide (silica) and tantalum pentoxide (tantala), which reflect radiation in certain wavelength regions by reason of the difference in index of refraction (1.46 and 2.2, respectively) between the two oxides. Production of such films is typically by successive evaporation of the oxides at low pressure and high temperature or by chemical vapor deposition, hereinafter "CVD", using tantalum alkoxides and tetraacyloxysilanes as reagents.
For ease of formation, the CVD operation is often carried out isothermally; i.e., at substantially the same temperature, typically about 500.degree. C., for the silicon and tantalum reagents. This temperature is not high enough for the deposition of silica in densified form. Instead, densification takes place spontaneously during operation of the article on which the film is deposited, causing stresses which may change the optical properties of the thin film or cause failure by way of peeling, cracking and the like. It is possible to use thermal cycling techniques to densify the silica prior to use, but this adds process steps and detrimentally affects the optical properties of the film,
An alternative to isothermal CVD production of optical thin films is deposition at two different temperatures, a relatively low one for the tantalum and a higher one for the silicon reagent. The low stress silica layers produced thereby are an advantage which, however, is counterbalanced by the disadvantage of required long deposition times and attendant high costs.